The specification relates to magnetic circuit design. In particular, the specification relates to magnetic circuit design for a speaker driver. FIGS. 1A through 1J depict a traditional magnetic circuit design for a speaker driver.
FIG. 1A is a cross sectional view 100 illustrating a traditional magnetic circuit design for a speaker driver. A traditional magnetic circuit design for a speaker driver includes a disc or ring-shaped magnet 104, a top plate 102, a traditional voice coil 110 and a yoke 106 as illustrated in FIG. 1A. The top plate 102 is a substantially circular disk-shaped object made of iron or low carbon steel and attached to the top of the magnet 104. The magnet 104 is disposed inside the yoke 106. The yoke 106 is a substantially circular bowl-shaped basket made of iron or low carbon steel. The top plate 102 and the magnet 104 are coupled into the yoke 106. For example, the top plate 102 and the magnet 104 are glued to the yoke 106 using a conventional adhesive. The space between the top plate 102 and the yoke 106 is referred to as a traditional magnet gap 108. The traditional voice coil 110 is coupled to a driver diaphragm and suspended in the traditional magnet gap 108.
The traditional magnetic circuit produces a magnetic field whose magnetic field lines 112a, 112b, 112c are illustrated in FIG. 1A. If an alternating current passes through the traditional voice coil 110, a Lorentz force is generated in response to the alternating current and the magnetic field. The Lorentz force acts on the traditional voice coil 110, causing the traditional voice coil 110 to move through the magnetic field. The direction of the Lorentz force is determined according to the right-hand rule. In other words, the direction of the Lorentz force is perpendicular to both the direction of the current in the traditional voice coil 110 and the direction of the magnetic field (e.g., the direction of the magnetic field lines 112a, 112b, 112c shown in FIG. 1A). Magnetic field lines 112a, 112b, 112c are referred to collectively as magnetic field lines 112.
FIG. 1B is another cross sectional view 120 illustrating a traditional magnetic circuit design for a speaker driver. As illustrated in FIG. 1B, a substantial portion of the magnetic field lines 112 that intersect the traditional voice coil 110 are not orthogonal to the longitudinal axis of the traditional voice coil 110 (e.g., the magnetic field lines 112 substantially deviates from a direction that is orthogonal to the {right arrow over (z)} direction as shown by the key in the top right-hand corner of FIG. 1B). This non-orthogonality of the magnetic field lines 112 causes the motion direction of the traditional voice coil 110 to deviate from an intended motion direction such as the ±{right arrow over (z)} direction, resulting in various distortions to the speaker driver as the traditional voice coil 110 moves.
For example, assume that a coil wire 130 wound around a former 132 has a changing current with a direction pointing out of the page (e.g., a direction pointing towards a user viewing the FIG. 1B). According to the right hand rule, a Lorentz force 134 is generated having a direction perpendicular to the direction of the current and the magnetic field line 112A. Because the magnetic field line 112A is not perpendicular to the longitudinal axis of the traditional voice coil 110 (e.g., the magnetic field line 112A is not perpendicular to the {right arrow over (z)} direction as shown in FIG. 1B), the direction of the generated Lorentz force 134 deviates from the {right arrow over (z)} direction. In other words, the Lorentz force 134 has a desired vertical component ({right arrow over (z)} component) parallel to the {right arrow over (z)} direction and an undesired horizontal component ({right arrow over (x)} component) orthogonal to the {right arrow over (z)} direction. This undesired horizontal component of the Lorentz force 134 causes the traditional voice coil 110 to bend and twist as the driver moves, leading to distortion in the driver system. The twisting of the traditional voice coil 110 can also cause the center dome of the driver diaphragm to expand and contract with the voice coil motion, which is referred to as a breathing mode. The breathing mode of the center dome of the driver diaphragm may introduce audible distortion during audio playback.
FIG. 1C is a graphical representation 140 illustrating angle variations of magnetic field lines from intersecting a traditional voice coil 110 at 90 degrees in a traditional magnetic circuit design. The graphical representation 140 is obtained using a conventional headphone driver. For example, FIG. 1C depicts the angle variations of magnetic fields lines from intersecting the traditional voice coil 110 at 90 degrees when the traditional voice coil 110 is stationary (e.g., the traditional voice coil 110 is at its rest position without any movement). FIG. 1C indicates that a substantial portion of the magnetic field lines is not perpendicular to the longitudinal axis of the traditional voice coil 110 when intersecting the traditional voice coil 110. For example, a substantial portion of the magnetic field lines intersects the traditional voice coil 110 at an angle substantially deviated from 90 degrees; this is represented by non-perpendicular region 197A, 197B. As described above, this non-orthogonality of the magnetic field lines causes various distortions such as twisting and bending of the traditional voice coil 110, a breathing mode, audio distortion, etc. Only a small portion of the magnetic field lines intersects the traditional voice coil 110 at an angle substantially perpendicular to the voice coil; this is represented by substantially perpendicular region 199. As a result, the design of FIG. 1C is limited in that the driver must be precisely mounted in the headphone driver housing.
FIG. 1D is a graphical representation 145 illustrating a contour plot of boundaries where the magnetic field lines have a deviation of ±3 degrees from intersecting a traditional voice coil 110 at 90 degrees in a traditional magnetic circuit design. The angle variations depicted in FIG. 1D are obtained from the angle variations depicted in FIG. 1C. Line 146A represents a boundary where the magnetic field lines have a deviation of −3 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, line 146A represents a boundary where the magnetic field lines intersect the longitudinal axis of the traditional voice coil 110 at 87 degrees. Lines 146B and 146C represent boundaries where the magnetic field lines have a deviation of ±3 degrees from intersecting a traditional voice coil 110 at 90 degrees. For example, lines 146B and 146C represent boundaries where the magnetic field lines intersect the longitudinal axis of the traditional voice coil 110 at 93 degrees.
In an area from line 146A to the left of the box 148, the magnetic field lines have a deviation of at least −3 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle less than 87 degrees. In an area from line 146B to the right of the box 148, the magnetic field lines have a deviation of at least +3 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle greater than 93 degrees. In an area from line 146C to the bottom of the box 148, the magnetic field lines have a deviation of at least +3 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle greater than 93 degrees. Thus, FIG. 1D indicates that a substantial portion of the magnetic field interesting the traditional voice coil 110 has a direction deviated by at least ±3 degrees from a direction perpendicular to the longitudinal axis of the traditional voice coil 110.
FIG. 1E is a graphical representation 150 illustrating a contour plot of boundaries where the magnetic field lines have a deviation of ±2 degrees from intersecting a traditional voice coil 110 at 90 degrees in a traditional magnetic circuit design. The angle variations depicted in FIG. 1E are obtained from the angle variations depicted in FIG. 1C. Line 152A represents a boundary where the magnetic field lines have a deviation of −2 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, line 152A represents a boundary where the magnetic field lines intersect the longitudinal axis of the traditional voice coil 110 at 88 degrees. Lines 152B and 152C represent boundaries where the magnetic field lines have a deviation of +2 degrees from intersecting a traditional voice coil 110 at 90 degrees. For example, lines 152B and 152C represent boundaries where the magnetic field lines intersect the longitudinal axis of the traditional voice coil 110 at 92 degrees.
In an area from line 152A to the left of the box 156, the magnetic field lines have a deviation of at least −2 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle less than 88 degrees. In an area from line 152B to the right of the box 156, the magnetic field lines have a deviation of at least +2 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle greater than 92 degrees. In an area from line 152C to the bottom of the box 156, the magnetic field lines have a deviation of at least +2 degrees from intersecting the traditional voice coil 110 at 90 degrees. For example, the magnetic field lines in this area intersect the longitudinal axis of the traditional voice coil 110 at an angle greater than 92 degrees. Thus, FIG. 1E indicates that a substantial portion of the magnetic field intersecting the traditional voice coil 110 has a direction deviated by at least ±2 degrees from a direction perpendicular to the longitudinal axis of the traditional voice coil 110.
FIG. 1F is a graphical representation 155 illustrating various locations (locations at lines 154A, 154B, 154C, 154D, 154E and 154F) where the magnitude of the magnetic field is measured. FIG. 1G is a graphical representation 160 illustrating the magnitude of the magnetic field at various locations illustrated by lines 154A, 154B, 154C, 154D, 154E and 154F, respectively. The graphical representation 160 is obtained using a conventional headphone driver. Line 164A depicts the magnitude of the magnetic field at line 154A. Line 164B depicts the magnitude of the magnetic field at line 154B. Line 164C depicts the magnitude of the magnetic field at line 154C. Line 164D depicts the magnitude of the magnetic field at line 154D. Line 164E depicts the magnitude of the magnetic field at line 154E. Line 164F depicts the magnitude of the magnetic field at line 154F.
The variations of the magnitude versus the distance as depicted by lines 164B and 164D indicate that there are substantial magnitude variations across the traditional magnet gap 108 from the top plate 102 to the yoke 106. Furthermore, the magnitude variations among individual lines 164A-164F indicate that there are substantial magnitude variations of the magnetic field intersecting the traditional voice coil 110. These magnitude variations cause unequal Lorentz forces to be generated and acting at different portions of the traditional voice coil 110. The unequal forces incur a torque on the traditional voice coil 110 and therefore expose the driver to rocking modes. The rocking modes occur when one side of the driver diaphragm lifts higher than the other side of the driver diaphragm. The rocking modes may incur audible distortion or a non-preferred frequency response curve for a driver.
The magnitude variations in FIG. 1G indicate the non-uniform magnetic flux density (or, non-uniform strength of the magnetic field) in the traditional magnet gap 108. The non-uniform strength of the magnetic field may exaggerate the voice coil misalignment problem. For example, in the assembly process it is possible that the traditional voice coil 110 is disposed out of its center position due to assembly errors. If the magnetic field close to the top plate 102 is stronger than the magnetic field close to the yoke 106, the voice coil misalignment may cause a first portion of the traditional voice coil 110 close to the top plate 102 to be exposed to a stronger magnetic field than a second portion of the traditional voice coil 110 close to the yoke 106. As a result, different portions of the traditional voice coil 110 are subjected to unequal forces because of the non-uniform strength of the magnetic field, which incurs undesirable rocking modes for the driver as described above.
FIGS. 1H-1J are graphical representations 170, 180, 185 illustrating the force acting on a traditional voice coil 110 in different sample times (0.000000 second, 0.000375 second, 0.001625 second) for a traditional magnetic circuit design. The graphical representations 170, 180 and 185 are obtained using a conventional driver. FIGS. 1H-1J indicate that the direction of the forces 172, 182, 187 acting on the traditional voice coil 110 substantially deviate from an intended motion direction of the traditional voice coil 110 (e.g., the direction of the forces 172, 182, 187 substantially deviate from a direction parallel with the longitudinal axis of the traditional voice coil 110), which causes various distortions in the driver as described above.
Generally, a traditional voice coil 110 in a traditional magnetic circuit design is coupled to a driver diaphragm using an adhesive and extends above the traditional magnet gap 108 as shown in FIGS. 1A, 1C-1F and 1H-1J. This overhung design approach exposes the upper portion of the traditional voice coil 110 to the stray magnetic field lines above the traditional magnet gap 108. Meanwhile, the lower portion of the traditional voice coil 110 is disposed in the traditional magnet gap 108, and exposed to a different magnetic field strength than the upper portion of the traditional voice coil 110. Thus, the traditional voice coil 110 is subjected to different magnetic field strength as a function of the voice coil position. This varying field strength interacting with the traditional voice coil 110 leads to compression in the voice coil motion, causing additional audio distortions.